1. Field of the Invention
The present invention relates generally to a door or window, and more particularly to a covering of a building's opening.
2. Description of the Related Art
A covering for an opening of a building, such as window or door, has various types. Take a window for example, the common type of the window has two sliding or hinged sashes. In this type of window, the sashes take some space of the window opening. It is bad for ventilation. Another window is similar to a casement window, having two hinged sashes, one of which is pivoted on the frame, and the other one of which is slidably engaged with a rail. Slide the sash will fold the sashes. In this window covering, the rail is straight, and the sash can't be moved when the folded sashes fold up. Therefore, people may hit the sashes when he/she walks by U.S. Pat. No. 1,570,958 disclosed a conventional door having a warp rail 1 as shown in FIG. 1. The rail 1 includes a straight section 1a and a curved section 1b. A first sash 2 is pivoted on a wall W, and a second sash 3 is hinged with the first sash 2. The second sash 3 is provided with a pulley assembly 4 to engage the rail 1. Just like the casement window, the second sash 3 is moved toward the first sash 2 to open the door. The sashes 2, 3 are folded, and moved off the opening because of the curved section 1b. It is easy to understand that the warp rail is expensive and difficult to manufacture. Furthermore, the warp rail makes the sashes move in a strange way. It is not good looking. In addition, while the sashes 2, 3 are opened, although they still are attached to the wall W (shown as the dot lines in the drawing), they are not tightly folded together.